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From "Tsz Wo \(Nicholas\), Sze" <s29752-hadoopu...@yahoo.com>
Subject Re: hflush not working for me?
Date Fri, 09 Oct 2009 22:27:59 GMT
Hi St.Ack,

HLog uses SequenceFile.Reader.  When a SequenceFile.Reader is constructed, it calls fs.getFileStatus(file).getLen().
 However, fs.getFileStatus(file).getLen() does not return the hflushed length for un-closed
file since the Namenode does not know the hflushed length.  DFSClient have to ask a datanode
for the length last block which is being written; see also HDFS-570.  That's why "hflush not
working for you".  Sorry ...

We have to change SequenceFile.Reader in order to support reading on unclosed SequenceFile.

Nicholas



From: "Tsz Wo (Nicholas), Sze" <s29752-hadoopuser@yahoo.com>
>To: hdfs-user@hadoop.apache.org
>Sent: Fri, October 9, 2009 2:40:34 PM
>Subject: Re: hflush not working for me?
>
>
>Soft lease is for another writer to obtain the file lease if the original writer appears
to abandon the file.  In the current TestReadWhileWriting (not counting part (c) and (d)),
there is only one writer.  So soft lease is not related.
>
>Will check your test.
>
>Nicholas
>
>
>
>From: stack <stack@duboce.net>
>>To: hdfs-user@hadoop.apache.org
>>Sent: Fri, October 9, 2009 2:32:02 PM
>>Subject: Re: hflush not working for me?
>>
>>>>On Fri, Oct 9, 2009 at 1:27 PM, Tsz Wo (Nicholas), Sze <s29752-hadoopuser@yahoo.com>
wrote:
>>
>>Hi St.Ack,
>>>
>>>> ... soft lease to 1 second ...
>>>You are right that you don't have to change soft lease.  It is for append but
not related to hflash.
>>>
>>>
>>
>>I should not have to set it then?  I can remove this 70 second pause in middle of
my test?
>>
>> 
>>
>>>> Do I have to do open as another user?
>>>This should not be necessary.
>>>
>>>Could you send me/post your test?
>>>
>>>
>>
>>Sure, as long as you don't hold this ugly code against me ever after.
>>
>>I checked in the code so you could try it:  http://svn.apache.org/repos/asf/hadoop/hbase/trunk/src/test/org/apache/hadoop/hbase/regionserver/TestHLog.java
>>
>>Its the first test, testSync.
>>
>>It starts out by copying whats down in the hdfs testReadWhileWriting.  That bit works
fine.
>>
>>Then comes the ugly stuff.
>>
>>HLog is our write-ahead log wrapper.  Internally it writes out to a SequenceFile.Writer.
 The SequenceFile.Writer has been doctored using reflection so the out datamember is non-private.
 A call to HLog.sync runs the SequenceFile.Writer.sync -- which DOES NOT call sync on the
backing output stream -- and then it calls sync on the now accessible out stream (Sorry its
so ugly -- I'm trying to hack stuff up fast so all of hbase gets access to this new facility).
 If I trace in the debugger, I can see that the sync on the out data member goes down into
hflush.  Queued up edits are flushed.  It seems like it should be working.
>>
>>Do I have to do some doctoring of the reader? (It doesn't seem so given that the code
at the head of this test works).
>>
>>Thanks for taking a look Nicholas.
>>
>>To run the test, you can do "ant clean jar test -Dtestcase=TestHLog".
>>
>>(Let me know if you want an eclipse .project + .classpath so you can get it up in
an ide to run debugger).
>>
>>St.Ack
>>
>>
>>
>> 
>>Nicholas Sze
>>>
>>>
>>>
>>>From: stack <stack@duboce.net>
>>>>To: hdfs-user@hadoop.apache.org
>>>>Sent: Fri, October 9, 2009 1:13:37 PM
>>>>Subject: hflush not working for me?
>>>>
>>>>
>>>>>>>>I'm putting together some unit tests up in our application
that exercise hflush.  I'm using minidfscluster and a jar made by building head of the 0.21
branch of hdfs (from about a minute ago).
>>>>
>>>>Code opens a file, writes a bunch of edits, invokes hflush (by calling sync
on DFSDataOutputStream instance) and then, without closing the Writer, opens a Reader on same
file.  This Reader does not see any edits not to mind edits up to the sync invocation.
>>>>
>>>>I can trace the code and see how on hflush it sends the queued packets of
edits.
>>>>
>>>>I studied TestReadWhileWriting.  I've set setBoolean("dfs.support.append",
true) before minidfscluster spins up.  I can't set soft lease to 1 second because not in same
package so I just wait out the default minute.  It doesn't seem to make a difference.
>>>>
>>>>Do I have to do open as another user?
>>>>
>>>>Thanks for any pointers,
>>>>St.Ack
>>>>
>>
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